For the purposes of imaging a scene, at low ambient illumination levels using a focal plane array comprised of solid-state detector devices, it is beneficial for the individual detectors in the array to be sensitive to light over a broad wavelength range and at single photon intensity levels. It would also be ideal if the focal plane array pixels were capable of providing their own source of short pulse, laser illumination to the area in the scene that is conjugated (using a camera lens for example) back to the respective pixel, providing the laser light pulse. Such an ideal solution would allow the solid-state detector pixels of the focal plane array to operate either in a conventional passive mode, collecting 2-D intensity images of a scene and also in an active mode, collecting 2-D or 3-D range images by detecting the time-of-flight of the optical returns reflected from objects in a scene that were illuminated by the short laser pulses generated by the pixels. The result is a highly compact, monolithic, all solid-state, focal plane array imager that supports functionality of a conventional, high resolution 2-D camera, as well as the functionality of a far less conventional, high resolution, 3-D camera, all on a single integrated circuit.
The present invention describes a method for implementing a compact, micrometer size, all solid-state emitter-detector pixel for large scale and high resolution, passive and active imaging focal plane arrays. The emitter-detector pixel consists of a silicon avalanche photodiode (APD) detector, electrically and optically integrated with a gallium nitride (GaN)/indium gallium nitride (InGaN) vertical-cavity surface-emitting laser (VCSEL) diode. The emitter-detector pixel is therefore capable of providing short pulse laser illumination at the pixel level, to the area in a scene, spatially conjugated (using a camera lens for example) back to the pixel providing the short pulse laser illumination. This method enables a compact, high resolution camera system, capable of passive, and efficient active mode imaging.
In the relatively recent past, it has become possible to fabricate solid-state arrays of silicon avalanche photodiodes optimized to operate either in linear mode or in non-linear Geiger-mode and capable of providing single photon sensitivity over a wavelength range from ultraviolet (UV) to near infrared (NIR). Focal plane arrays comprised of such linear or Geiger-mode silicon APD pixels, however, could only detect light with high sensitivity and were not in turn able to provide their own short pulse laser illumination at the pixel level, to an area in the scene spatially conjugated (using a camera lens for example) back to the respective pixel providing the illumination. To date, no effective technology is available to allow the optical and electrical integration of a solid-state, laser light emitter with a linear or Geiger-mode avalanche photodiode at the micrometer dimensions required for high resolution, passive or active imaging solid-state focal plane arrays.
As illustrated in U.S. Pat. No. 7,271,376 the silicon avalanche photodiode is designed as a pure detecting element and does not provide a means for generation of short pulse laser light for illuminating a scene.
As illustrated in U.S. Pat. No. 7,268,399 a method is described for forming semiconductor avalanche photodiodes in silicon using a plurality of doped, opposing trenches in the top and bottom surfaces of the substrate wafer. The detection device described in the invention makes no provision for integrating a solid-state laser with the radiation detecting elements.
As illustrated in U.S. Pat. No. 6,864,965, the imaging focal plane array supports dual-mode operation in both passive and active detection modes using LADAR pulses for the active mode. Switching between detection modes is accomplished by increasing the voltage bias across the detector so as to increase the gain and therefore sensitivity to the active laser pulse returns from objects in a scene. The detector pixels in the focal plane array, however, do not contain a solid-state laser light source at the pixel level, for purpose of illuminating the area in the scene that is spatially conjugated (using a camera lens for example) back to the illuminating pixel, in active detection mode.
In another embodiment of a position sensitive solid-state detector with internal gain, U.S. Pat. No. 6,781,133 B2, the invention describes a detection device and signal readout scheme, yet no provision is made for a solid-state laser light emitter to be integrated with the detector device, that would support the high resolution, active imaging function.
In another embodiment, U.S. Pat. No. 5,892,575, a method and apparatus for imaging a scene are described for resolving the 3-D spatial structure in the scene. The light source emits pulses of laser light toward the object being imaged and the detector system includes an optical system and an array of light detectors operating in non-linear Geiger-mode. The optical system collects a portion of the light scattered off of objects in the scene and directs the collected light toward the array of light detectors. The invention refers to a monolithic array of light detectors operating in the non-linear Geiger-mode and does not describe a method for an all solid-state, monolithic detector array that combines light detectors with solid-state laser light emitters at the micrometer scale, pixel level.
In another embodiment, U.S. Pat. No. 5,757,057, a method for fabricating a large array of avalanche photodiodes using a plurality of pixel contacts that are isolated by one or more isolation structures is revealed. The avalanche photodiode pixels, however, do not contain a monolithically integrated solid-state laser light emitter.
In another embodiment, U.S. Pat. No. 5,438,217, a planar avalanche photodiode device array is realized using a planar block of n-type semiconductor having a plurality of p-type wells in the block surrounded by a foundation of n-type semiconductor material. The individual solid-state detector elements of the array do not have an integrated solid-state laser light emitter.
Note that the above solid-state, semiconductor, avalanche detectors do not envision, nor describe a method for realizing a single monolithic detector pixel forming part of a large, imaging focal plane array, that combines emitter-detector functionality at the micrometer scale, pixel level.